It is well known that optical feedback in a laser system produces variations in output power, excess noise, and number of other undesirable effects which degrade system performance. The ability to measure optical feedback at various points in a laser system would enable laser system architects to improve the design and construction of their laser systems and thus to enhance system performance by optimizing signal-to-noise ratio.
A method for measuring optical feedback in an index-guided single-mode semiconductor laser is discussed in the article The Influence of Feedback Intensity on Longitudinal Mode Properties and Optical Noise in Index-Guided Semiconductor Lasers by Acket, Lenstra, Boef, and Verbeek published in IEEE Journal of Quantum Electronics, Vol. QE-20, No. 10, October 1984. This method involves 1) using a scanning Fabry-Perot interferometer to measure the amount of feedback-induced frequency shift in the semiconductor laser beam and 2) determining the relative strength of the optical feedback by calculating a parameter which is a function of the feedback-induced frequency shift and the external cavity length.
This method has at least one drawback--its use of a scanning Fabry-Perot interferometer increases significantly the number of computations required to measure the frequency shift induced in the laser beam, which in turn slows the rate at which an optical feedback measurement can be provided.
A need exists for a method and apparatus for measuring optical feedback in a laser system which represents a substantial improvement over known methods, and more specifically which is capable of providing a near real time optical feedback measurement at various preselected locations in a laser system.